Monday, August 6, 2012

Understanding the SysRq Key – The Magic Key To Control Linux

http://maketecheasier.com/the-usage-of-sysrq-key-in-linux/2012/07/31

Linux do crash. It is not so frequent, but it happens. You know what to do when an application refuses to obey, but if everything fails, the last solution is using the supreme power on Linux: the magic SysRq
key. By using this key with a combination of “Alt” and another letter,
you will be able to make your computer respond no matter what. It was
already evoked, but a little explanation does not hurt, and the more you
know, the more prepared you will be when facing a real problem. Also,
understanding the individual effect is always better than just repeating
a combo learned by heart.

How To Activate The Magic Key

As
said before, the magic key is super handy. But first, you have to
activate this functionality if it is not already done. To check, use the
command

cat/proc/sys/kernel/sysrq

If
it returns a 1, the Magic Key is working. If it is a 0, you will need
to activate it. In most cases, if you want to activate it temporarily,
you can try

sudoecho"1">/proc/sys/kernel/sysrq

But this did not work for me. So under Archlinux, I used instead:

sudo sysctl kernel.sysrq=1

Note
that this will be only temporary. If you want the Magic Key to work
even after a reboot, the best way is to edit the file
“/etc/sysctl.conf”, so that the line

kernel.sysrq = 0

becomes

kernel.sysrq = 1

List of Useful Combinations

The
whole point of using the Magic SysRq key is to combine it with the
“Alt” key and another letter in order to trigger a particular function,
even if your computer seems frozen. To begin, you should know that this
System Request key is usually sharing its location with the “Print Scrn”
key at the top right of your keyboard. Above the “Insert” key, and left
to “Scroll Lock.”
As
you can guess, the effect of the combination depends on which letter
key is used. Here is a non-exhaustive list of letter you can use to
obtain an interesting result.

Famous Examples

Every
Linux user should know these examples. If your X server is frozen and
you cannot access any virtual console, using “Alt + SysRq + r” should
take the keyboard away from the X server and let you perform “Ctrl + Alt
+ F1″ in order to get to a terminal and kill the appropriate processes.
If
you do not have the patience to do that, “Alt + SysRq + k” will kill
everything in the same way as “Ctrl + Alt + Backspace” (but more
violently).
If nothing responds at all, and the only way is to reboot your computer, the classical combo is “reisub” for “Raising Elephant Is So Utterly Boring”, or just remember the opposite of “busier“. Use each letter of this anagram with the magic key in order to:

Take the keyboard from X

Ask all programs to end gently

Kill the one who did not

Save the data from the cache to the hard drive

Remounts the file systems

Reboot

Conclusion

The
idea is that the magic key combinations are generally super violent.
You do not want anybody to use it on your computer while you are not
looking. But as a last resort, it can become the ultimate tool. I rarely
encountered a situation where SysRq did not respond. So when your Linux
does not obey its master, you know what to do.
Do you know any
other combination involving SysRq? Or some other example? Or do you have
any questions? Please let us know in the comments.
Image credit: SysRq